This disclosure relates to fenestrated bone screws, and in particular to fenestrated pedicle bone screws, and to methods of installing such fenestrated bone screws in a variety of surgeries, particularly spine surgeries, and to the methods of injecting bone cement (e.g., polymethyl methacrylate (PMMA), or the like) into the proximate bone structure into which such fenestrated screws are installed.
In many instances, particularly in spine surgeries, the patient may have diminished or osteoporotic bone quality, which lessens the purchase of bone screws in such bone structure. In spine surgery, such diminished bone quality oftentimes precludes stabilization of the vertebrae and leads to pedicle screw loosening and pullout.
In order to enhance fixation of bone screws in osteoporotic bone, multiple solutions have been tried. Pedicle screw design has been optimized including greater thread depth, decreased thread cross-sectional thickness, buttress thread design, greater purchase length, and maximized major diameter. Greater insight into insertion technique by the surgeon has been achieved. Smaller diameter pilot holes, undertapping, and avoidance of multiple passes are standard technique as well as fixating more levels to distribute loads more widely. Pedicle screw surface coating with hydroxyapatite results in increased pullout resistance and decreased loosening by promoting bonding between bone and the apatite coating of the screw. Unfortunately these screws are expensive and sub-optimally perform in biologically challenged osteoporotic bone.
Kyphoplasty augmented pedicle screw fixation represents an initial attempt at utilizing the injection of bone cement to increase pedicle screw pullout strength. Kyphoplasty is typically performed by the injection of 2-3 ml. of bone cement into a void in the bone structure formed by the inflation of a special balloon into the vertebrae. As the balloon inflates, it compacts the soft inner bone structure to create a void or cavity inside of the vertebrae. The balloon is then removed and bone cement is injected into the cavity. However, if such procedure is used to enhance the purchase of, for example, a pedicle screw to be inserted into the vertebrae, the screw must be inserted before the cement has hardened.
While screw loosening and the tendency of the screw to pull out of the bone are reduced, percutaneous screw placement is not possible and thus kyphoplasty cannot be used with minimal invasive surgery (MIS) platforms. Further, once the screw is placed, it cannot be repositioned if the screw is inserted along an unsatisfactory trajectory. Still further, if multi-level fixation is required, such kyphoplasty augmented screw placement would require multiple kyphoplasty kits (e.g., balloons and injection systems) that would make the procedure prohibitively expensive.
Further technological improvements have resulted in the development of various systems using cannulated/fenestrated bone screws for injecting bone cement into the bone structure into which the fenestrated bone screw has been inserted. Such cannulated/fenestrated screws typically have a central bore or cannula extending lengthwise of the screw along its central axis with the cannula extending through the tip of the screw. One or more side bores or fenestrations are typically located along the length of the bone screw in the area of the bone structure into which it is desired to inject the bone cement. The number of fenestrations may vary, but typically there may be between 3 and 6 fenestrations in the screw, depending on the length of the screw. Such screws are commercially available in various lengths and diameters.
In spine surgeries, such fenestrated bone screws may be used in either minimum invasive surgeries (MIS) or in open surgical procedures. The placement of the screws in the vertebrae or pedicles is carried out using known techniques for targeting and placement of the screws. A cement delivery cannula is connected to the cannulated screw. A cement pump is connected to the delivery cannula and the surgeon pumps bone cement (preferably a radio opaque cement) through the delivery cannula into the cannulated bone screw, out the fenestrations and the distal end of a fully cannulated screw into the surrounding bone structure before the cement has hardened. Fluoroscopy is used monitor the injection or extrusion of the cement into the bone structure. Once the cement has been injected into the bone structure, the delivery cannula is removed. Of course, once the cement has set up, enhanced screw purchase and stability of the bone structure results.
While such prior cannulated/fenestrated screw and cement injection systems have worked for their intended purpose, deficiencies have been found. The coupling and uncoupling of the cement injection system to the screw head of such screw bone cement augmentation systems has been problematic. Typically, the attachment of the cement injection cannula to the screw head has not been mechanically secure such that the surgeon oftentimes was required to hold the cement injection cannula docked to the screw head and to monitor for cement leakage Particularly in MIS procedures, the screw head is not visible to the surgeon such that it is difficult to reversibly mechanically connect the cement injection cannula to the screw head.
Still further, with such prior art cannulated/fenestrated screws, if the cannula fully extended to and through the tip of the screw, upon insertion of the screw into the bone structure, a bone core would enter the cannula of screw with final positioning of the screw in the desired anterior one third of the vertebral body. This bone core would block or occlude the flow of cement in the main channel of the screw and would block the flow of cement to the fenestrations or side bores effectively rendering the desired function useless. As a result, in such prior cannulated/fenestrated pedicle screws, the location of the injected bone cement is potentially along the posterior half of the screw in the posterior of the vertebrae, which is a far from optimal because of the potential extravasation of the bone cement into the spinal canal and foramen.
There are other prior systems where a cement delivery cannula is inserted into the axial bore or central cannula of the fenestrated screw. Of course, in order to enable the delivery cannula to be inserted into the central bore of the screw, there must be sufficient clearance between the inside diameter of the central bore of the screw and the outside diameter of the delivery cannula. Further, such cement delivery cannulas are typically elongate, small diameter tubes, which provide significant resistance to the flow of the cement for efficient injection of cement.